The insulin receptor is solubilized from human placental membranes. Purification of the solubilized insulin receptor is being undertaken using anti-receptor antibody. An immunoprecipitation method using the 125I-insulin receptor complex has been developed to assay for insulin-receptor antibodies. Specificity of receptor antibodies can be used to measure receptor concentration. Characterization of the solubilized insulin receptor on IM-9 cultured human cells is also being studied.